1. Field of the Invention
This invention relates to a color image forming apparatus adapted to form a toner image on a belt-shaped image retainer by an electrophotographic system, to transfer the toner image on a transfer material and to obtain an object image.
2. Description of the Prior Art
Many methods and apparatuses for obtaining color images by using electrophotography have been proposed. As disclosed in, for example, Japanese Patent Laid-open No. 100770/1986, there is a method of obtaining a color copy by forming a latent image on a photosensitive drum serving as an image retainer in accordance with a color separation number of a document image, developing the latent image on the drum, transferring the resultant image onto a transfer drum every time the development of the latent image is completed to form a multi-color image on the transfer drum, and thereafter transferring the multi-color image onto a transfer material to obtain a color copy. An apparatus realizing this method needs to be provided with a large transfer drum where a one-frame image is transferred on the circumferential surface in addition to a photosensitive drum. Consequently, it is unavoidable that this apparatus has a large and complicated structure.
There is another method disclosed in, for example, Japanese Patent Laid-open No. 149972/1986, which comprises the steps of forming a latent image on a photosensitive drum in accordance with the color separation number of a document image, developing the latent image on the drum, and transferring the resultant image onto a transfer material every time the development of the latent image is completed, to obtain a multi-color copy. In this method, it is difficult to accurately superpose each of monochromatic images upon one another, so that a high-quality multi-color copy cannot be obtained.
There is also another method of obtaining a color image, which comprises the steps of repeating formation of a latent image on a photosensitive drum in accordance with the color separation number of a document image and development of the latent image with color toners, superposing color toner images upon one another on the photosensitive drum, and then transferring the resultant image onto a transfer material. The basic processes for formation of such a multi-color image are disclosed, for example, in Japanese Patent Laid-open Nos. 75850/1985, 76766/1985, 95456/1985, 95458/1985 and 158475/1985.
In a multi-color image forming apparatus for obtaining a color image by such an image superposing step, a plurality of developing devices storing different color toners therein are arranged around a photosensitive drum, and a latent image on this drum is developed by rotating the same drum generally a plurality of times to obtain a color image.
In addition to the photosensitive drum which has a photoconductive material applied or evaporated on the circumferential surface of the drum as described above, a belt-shaped image retainer which has a photoconductive material applied or attached to a flexible belt has also been proposed. Since the shape of a belt-shaped image retainer is determined by training the same around rollers including a driving roller, such an image retainer is an effective device for making a compact color image forming apparatus by utilizing the space advantageously. Since the photosensitive belt can be moved along a line of a small curvature, the imperfect separation of a transfer material can be prevented by utilizing a curved portion of a small-diameter roller in use.
In a color image forming apparatus using a photosensitive belt described above, the image forming means including a charging means, an exposure means, a plurality of developing devices and a cleaning means are provided where they faces mainly the lower portion of the outer circumferential surface of the photosensitive belt. These image forming means including the photosensitive belt are incorporated in a unit as a process cartridge and the unit is detachably housed independently in an apparatus body.
The transfer material with a color toner image transferred thereon is ejected, with its copied surface down, onto a receiving tray placed on the upper surface of the apparatus, and the apparatus can be installed in a small space.
It has been needed to use a color image forming apparatus of the type described above in which as a transfer material, thick transfer materials such as transparent sheets for OHP or postcards besides regular transfer paper can be used. When thick transfer material is bent along a surface of a large curvature, it would be curled stiffly, and creased so that the image thereon would be damaged. Therefore, it is necessary that the passage for the transfer paper has a small curvature. Accordingly, the radius of curvature of the passage in the discharge section extending toward the tray has to be set large. However, when this radius of curvature is set large, the leading end of the regular transfer paper turns inward during the ejection of the same, and it is not properly stacked on the tray.
In order to solve this problem, it is considered that a receiving tray for thick transfer material is provided on a separate portion of the apparatus so that the passage from the transfer paper entry section to the tray is formed on a straight line. However, in such an apparatus, necessarily the thick and thin transfer material are ejected separately to different portions of the apparatus, with the copied surfaces directed in different directions, i.e. up and down. Consequently, the size of the apparatus increases, and a wide space for installing the apparatus is required. Moreover, the operation of the apparatus becomes complicated.
When the transfer material of different thickness is used, the fixing of an image is not done perfectly due to different required quantities of heat, and the image may be offset fixed.